Size and shape in cranial evolution of 2 marsupial genera:DidelphisandPhilander(Didelphimorphia, Didelphidae)

Abstract

Ontogeny is an important source of morphological changes. The combination of size and shape analyses throughout ontogeny helps us to better understand the importance of size and shape from an evolutionary and developmental point of view. Combining geometric and traditional morphometric analyses, we compared skull ontogenetic trajectories of 2 New World didelphimorphian marsupials, Didelphis and Philander. We obtained 32 landmarks and 35 Euclidean distances between these landmarks. Despite secondary sexual dimorphism in both genera, we obtained similar results for both sexes. Allometric comparisons showed similar skull growth patterns and rates. Most of the skull regions had negative allometric growth, except for the zygomatic region. The positive allometric growth of the zygomatic region is possibly related to the transition from lactation to solid food. We investigated size influence in separating genera, comparing analyses based on original and size-free data. Although original data separated both genera, size-free data failed to do so. We found a significant positive correlation between character changes (shape) and size when analyzing growing trajectories, suggesting an allometric change pattern. Geometric morphometrics also corroborated the notion of generic differences being mainly size-related (either scale or allometry). Vectors describing the ontogenetic trajectory were highly similar, both between genera and within each genus throughout ontogeny. Didelphis and Philander share the same orientation in their ontogenetic trajectories because of similar growth rates. However, Didelphis trajectories are relatively longer, indicating an extended growth period. Isometric size changes are responsible for 98.36% of all differences between genera in adult specimens, whereas 1.43% of differences is due to allometric shape variations, and 0.21% is due exclusively to shape variation.